The invention pertains to a method of increasing the sulfuric acid concentration of a sulfuric acid solution.
Sulfuric acid solutions are used in various chemical processes. One of these processes is the manufacture of aramid polymer. As an example, the manufacture of aramid fiber is mentioned.
Aramid fiber is normally produced by dissolving PPTA (para-phenylene terephthalamide) or another aramid polymer in 100% sulfuric acid after which the resulting spin dope is expelled through a spinneret. Sulfuric acid is then removed from the fibers in a washing step using water, resulting in a diluted solution of approximately 10-15% sulfuric acid (H2SO4).
The diluted sulfuric acid solution can be neutralized and disregarded, resulting in a considerable emission of sulfates and the use of considerable amounts of sulfuric acid and water from an external source. Therefore, it is more convenient to recycle it. For instance, it can be concentrated to 96% sulfuric acid in a series of different distillation processes. Such process is described in WO 2009/053254.
According to the method of WO 2009/053254 a sulfuric acid recovery unit has been used wherein the sulfuric acid concentration of diluted sulfuric acid is stepwise increased to obtain 96% sulfuric acid.
In a step of this process 78% sulfuric acid is first concentrated to 85% sulfuric acid, and then in a next step further concentrated to 96% sulfuric acid. This further concentration step is performed in an apparatus with inlet and outlet comprising an evaporator vessel, pump, and heaters. These elements are connected to each other by an enameled piping system. It was now experienced that after longer periods of operating serious problems occur in the piping system. The piping is enameled at its inner surface. This is necessary because sulfuric acid/water and sulfuric acid/hydrogen peroxide mixtures are extremely corrosive at elevated temperatures. The only system that can be used for industrial application are pipes, usually steel pipes, with a few millimeters thick enamel coating at their inner surfaces. It now appears that within a period of a few months the shiny enamel becomes dull. As such this is not a serious problem, but after a much longer period, which can be as long as about 3 years of operating, it appears that the enamel is seriously damaged, and even more seriously can completely be disappeared. The positions where the enamel has disappeared are called cracks. The sulfuric acid will then directly contact the steel at those cracks and will corrode the steel piping. Although this piping system has been made of 8 mm thick steel, sulfuric acid completely dissolves the steel within 4 hours. If this happens a hot stream (about 240° C.) of sulfuric acid will be sprayed over the site, which obviously can lead to life threatening situations.
Up to now the only remedy and prevention of this hazardous situation is the regular inspection of the piping and the timely replacement of the enamel layer. This is however and extremely expensive operation, which costs in the order of about 500 thousand euro. It is therefore of great importance to find a solution to prevent completely the impairment of the enamel layer.
In U.S. Pat. No. 4,274,910 (Forter) a method has been disclosed for preventing enamel cracks in the enamel of the heat exchanger. However, these cracks do not occur as the consequence of using concentrated sulfuric acid/hydrogen peroxide mixtures, but these cracks are the result of decreased “compressive stress” between the steel piping and the enamel coating by temperature differences during the heat exchange process. Compressive stress is required to prevent crack forming. As described by Forter, column 1, lines 17-49, this problem typically occurs in heat exchangers when uneven cooling and heating takes place. Forter has solved this problem by providing the heat exchanger with a jacket for circulating a heating medium thereby diminishing the temperature fluctuations in the heat exchanger. The Forter apparatus is suitable for making concentrated sulfuric acid from diluted sulfuric acid, such as explained in the examples where 75% sulfuric acid is obtained from 41.5% sulfuric acid, and thereafter the 75% sulfuric acid is further concentrated to 98% sulfuric acid. The present invention does not relate to preventing cracks in the heat exchanger but to solve the problem of occurring cracks in the evaporator vessel and pipelines, which problem does not occur when using the sulfuric acid concentrations of Forter. Thus in Forter's apparatus it is not required to provide the evaporator vessel with an enamel layer (column 4, lines 3-6). When making highly concentrated sulfuric acid/hydrogen peroxide mixtures, which mixture is much more corrosive than sulfuric acid alone, the use of an enameled evaporator vessel and enameled piping is necessary and without the application of this invention cracks will be formed when using the aggressive sulfuric acid/hydrogen peroxide mixture.